4 Answers
4

Programs like Audacity let you create a "noise profile" which is then used to remove noise on the recording. If you can find a few seconds where the only sound is the chalk-board noise, then you can try that.

IME, noise reduction works better in multiple passes than trying to catch it all in one pass.

The most annoying part of the chalk is probably the higher frequencies (squeaking), so a simple EQ pass might be able to make the recording more tolerable. Audacity has a few plugins that do EQ. A bit of trial and error and you should be able to quiet the highest frequencies while leaving the voice audible.